1. Field of the Invention
The present invention generally relates to a method of detecting a phase difference of image bearing members and an image forming apparatus using the method. More particularly, the present invention relates to a method of detecting a phase difference caused by velocity fluctuations of image bearing members and an image forming apparatus using the method.
2. Discussion of the Background
In general, it is known that an image forming apparatus having a plurality of image bearing members may have a phase difference of toner images formed on the plurality of respective image bearing members when the toner images are sequentially overlaid onto an image transferring member or directly onto a recording medium. This is because fluctuations in the rotational speed of respective image bearing members may occur.
In some image forming apparatuses, the phase difference caused by the fluctuations in the rotational speed of the image bearing members is calculated based on patterns formed on the image transferring member so that the rotational speed of the image bearing members can be controlled to compensate for the phase difference. The positions of the patterns, however, may change when the patterns are formed because a deviation in thickness of the image transferring member can occur. This may result in degradation in accuracy of phase adjustment or phasing.
In different image forming apparatuses, marks are previously formed on an image transferring member to detect the position of the marks by a mark detecting sensor. However, additional costs may be required to provide such marks and the mark detecting sensor in a non-image forming portion of the image forming apparatus.
One way to effectively adjust color shifts of image bearing members is for an image forming apparatus to repeatedly form combinations of registration patterns of the respective colors so that a formation area thereof may be equal to one rotation portion of the image transferring member. The registration patterns are detected for respectively obtaining data of color shifts of cyan, magenta, and yellow images with respect to a black image by a same amount as a circumference of the image transferring member. According to the data of color shifts, components derived from rotational irregularity of an image bearing member and components derived from traveling irregularity of an image transferring member are extracted and then stored. In an image forming operation, phases of the image bearing member and the image transferring member are detected, and the data of color shifts having the above-described components are made in alignment with phases thereof. To eliminate the aligned color shifts, a correction pulse compensating a writing timing for each scanning line to the image bearing member of each color is generated. According to the correction pulse, respective LED arrays are driven.
Another way is to calculate a mean value of deviations of color marks. Color images are formed on respective image bearing members and transferred to a transfer sheet in an overlaying manner. As for the detection of the color shift in color image formation, a plurality of mark sets including the array of respective color marks arranged in the moving direction of an image transferring member are formed on the image transferring member. The respective color marks of the respective mark sets are detected by sensors so that the average value of the deviation of the same color marks on the different mark sets from respectively corresponding reference positions can be calculated. In such detection, the plurality of mark sets are formed within the range of one circumferential length of the image transferring member. The same color marks on the different mark sets are formed at the pitch of three fourth (¾) circumferential length of the image bearing member. The number of sets to be formed is eight or four. Only analog-to-digital (A/D) conversion data within the range from approximately 2V to approximately 3V is stored in association with respective scanning positions into a memory, and center points of the marks are calculated.
In the full-color copying machine of four-series tandem type, a correction value is set based on the phase difference of velocity change for one circumferential length of the image bearing member which is determined based on the halftone band of uniform density for each color recorded in a recording medium. Based on the correction value, the rotational speed of the image bearing member is controlled, that is, increased or decreased. Thereby, the phase of the velocity change for one circumferential length of the image bearing member is adjusted.
The above-described techniques used in the respective image forming apparatuses, however, require complex controlling and are associated with insufficient prevention for deterioration in accuracy of phase adjustment caused by deviations due to thickness in the image transferring member.